As the elements in semiconductor devices have been highly-integrated, the interconnections in the elements are required to be arranged with high precision of submicron unit. In such elements, if fine particles and/or bacteria attach to the interconnections, the interconnections will be short-circuited, which will immediately cause product failure. Therefore, gas and wash water used in the manufacturing of semiconductor devices are required to have extremely high purity. It is also required to protect inner walls of vacuum chamber, components for reaction room such as electrodes and a gas introduction pipe from generation of impurity gas and fine particles.
Under such conditions, it is recommended that a vacuum chamber is made of stainless steel and aluminum alloy, because stainless steel and aluminum alloy could release only small amount of gas and has excellent corrosion resistance in general. However, even if the vacuum chamber is made of the stainless steel and aluminum alloy, it is difficult to avoid the corrosion by the halogen-containing gas used as a reaction gas or an etching gas and halogen-containing plasma generated from the halogen-containing gas. In an attempt to solve this problem, it has been suggested to form a coating film including TiN, AlN, Al.sub.2 O.sub.3 and the like as an essential material which exhibits excellent resistance to halogen-containing gas corrosion and halogen-containing plasma corrosion (see Japanese Utility Model Publication No. 61-13555, Japanese Laid-Open Patent Publication No. 1-312088, Japanese patent Publication No. 5-53871). As to the stainless steel, some technologies have been suggested to avoid its corrosion. For example, austenitic stainless steel is subjected to electrolytic polishing, and then is heated in oxidizing gas atmosphere. Accordingly, an amorphous oxide coating film is formed on the surface thereof, so that the surface releases only suppressed amount of gas (see Japanese Laid-Open Patent Publication No. 64-87760). In another technology, the content of non-metal inclusions is reduced to the value as small as possible, which may otherwise produce fine particles or may adsorb or release impurities (see Japanese Laid-Open Patent Publication No. 63-161145).
However, the properties of the coating film including TiN, AlN, Al.sub.2 O.sub.3 and the like as an essential material depend on its production conditions, and the resistance to halogen-containing gas corrosion greatly depends on the properties of the coating film. It is difficult to provide a coating film which always exhibits stable and excellent resistance to a hydrogen chloride gas, a hydrogen fluoride gas, which has extremely high corrosiveness, and halogen-containing plasma. In addition, the above-described technology for stainless steel cannot achieve sufficient corrosion resistance in such highly corrosive environment. When corrosion starts, various problems arise. For example, corrosion products adsorb and release the gas, and in addition, the products themselves turn into fine particles and attach to the inner surface of the vacuum apparatus or the surface of the semiconductor wafer treated therein. As a result, the inside the chamber and the surface of the semiconductor wafer are contaminated.
Next, among the processes of manufacturing of semiconductor devices, it is known that a process of chemical vapor deposition in which a halogen-containing gas and halogen-containing plasma are used.
A chemical vapor deposition apparatus is categorized in some types in accordance with the methods for allowing the gas introduced into the vacuum chamber to decompose or react. Examples of chemical vapor deposition apparatus include a thermal chemical vapor deposition apparatus, a plasma chemical vapor deposition apparatus, an optical vapor deposition apparatus, and the combination thereof such as a thermal and plasma chemical vapor deposition apparatus. Hereinafter, a thermal and plasma chemical vapor deposition apparatus will be described as a typical example. As shown in FIG. 1, a lamp heater 3 is disposed below a chamber (a reaction chamber) 1, so that a susceptor 6 is heated through a window 2 made of transparent ceramics such as transparent quartz glass or material consisted with transparent glass composition. On the susceptor 6, a semiconductor wafer 7 is disposed so as to be heated to, for example, 450.degree. C. by heat transferred from the susceptor 6. The air in the chamber 1 is discharged through an outlet 5, so that inside the chamber is kept under vacuum or under depressed pressure. Simultaneously, through an inlet 4, a material gas such as WF.sub.6 gas or an etching gas is introduced into the chamber 1. In this state, a high frequency voltage is applied between the susceptor 6 and the chamber wall, so that plasma is generated from the material gas. In this process, a coating film with a desired composition such as a tungsten coating film is formed on the semiconductor wafer 7. In other words, the heat is transferred from the lamp heater 3 to inside the chamber 1 through the window 2, so that a material gas is decomposed, the reaction is started and promoted, the formation of a coating film on the semiconductor wafer 7 is promoted, the structure of the coating film is improved, and the adhesiveness of the coating film onto the semiconductor wafer 7 is enhanced.
In order to allow light (mainly an infrared ray) from the lamp heater 3 to efficiently penetrate through the window 2, the window 2 is required to be transparent. Therefore, the window 2 is made of material with high transparency. The window 2 is also required to be heat-resisting because its surface facing inside the chamber 1 is exposed to heat at high temperature. In addition, the window 2 is required to have resistance to halogen-containing gas and halogen-containing plasma for the aforementioned reasons.
Hereinafter, the halogen-containing gas corrosion and the halogen-containing plasma corrosion will be further described in detail by taking the chemical vapor deposition apparatus as an example.
Typical examples of halogen-containing gases include an F.sub.2 gas and a WF.sub.6 gas. After a coating film is formed, an etching gas containing NF.sub.3 and the like may be introduced inside the chamber for cleaning purpose in some cases. Such a halogen-containing gas generates plasma during the formation or etching of the coating film, so that the surface of the window facing inside the chamber is exposed to the plasma. The window will be damaged or corroded if it is made of transparent ceramics, especially made of quartz glass solely made of SiO.sub.2 having low resistance to halogen-containing gas corrosion and halogen-containing plasma corrosion.
If the inner surface of the window is damaged or corroded, the transparency of the window will be deteriorated even if the damage or corrosion is small. Such a window is not preferable to be used in the chamber. If the window is damaged or corroded more seriously, localized pit-like corrosion occurs, and cracks are generated therefrom, and in addition, the total thickness of the window becomes small. As a result, a vacuum apparatus with such a window cannot be continuously used for maintenance reasons. There are also some cases where SiO.sub.2 is peeled-off from the window surface due to the corrosion. The removed SiO.sub.2 floats inside the chamber forming fine particles and attach to the surface of the semiconductor wafer. This may cause a large number of failure on semiconductor devices.
Under such circumstances, it is necessary to prevent the window material from being corroded by halogen-containing plasma. For this purpose, the use of transparent material containing no SiO.sub.2 can be considered. For example, plastic materials satisfy the requirement of transparency; however, there are problems in that their heat resistance is low and gas is released therefrom, and therefore, is not adequate for a window material for a chamber of a heat and plasma chemical vapor deposition apparatus. Instead of plastic materials, the use of ceramic material containing no SiO.sub.2 is also considered, because their heat resistance is high and small amount of gas is released therefrom. In this case, the ceramics material is required to be singlecrystalline or amorphous, because the polycrystalline ceramic material has problems in its transparency. However, both of them will have problems in their productions. That is, it is very difficult to produce a window material made of singlecrystalline ceramic having large diameter and large thickness. Especially, in a chemical vapor deposition apparatus, recently, a wafer of 8 inches or more in diameter is mainly treated. Such a wafer requires the chemical vapor deposition apparatus to have a window of more than 8 inches in diameter. Therefore, it is not practical to develop the window material made of singlecrystalline ceramic. On the other hand, production of the amorphous ceramic also has a problem in its thermodynamic point of view, and it is extremely difficult to produce an amorphous window material large in diameter.
In general, in the chemical vapor deposition apparatus, a process of producing a coating film and cleaning is repeated. In accordance with the conditions of this process, heating and cooling are repeated within a range between the room temperature and 450.degree. C. inside the chamber 1. At this time, the window material, which is a component of the chamber 1, is also subjected to the repeated heating and cooling.
In this case, even if a coating film including Al.sub.2 O.sub.3 is formed on a quartz glass to achieve the resistance to halogen-containing gas corrosion and halogen-containing plasma corrosion, the Al.sub.2 O.sub.3 coating film may be cracked or partially peeled off. This is because the quartz glass has thermal expansion coefficient of about 5.times.10.sup.-7 /.degree.C. and Al.sub.2 O.sub.3 has that of about 8.times.10.sup.-6 /.degree.C. The difference is 10 times or more. This large difference produces large thermal stress during the heating and cooling, which is then induced onto the Al.sub.2 O.sub.3 coating film. When the Al.sub.2 O.sub.3 coating film becomes cracked, a halogen-containing gas penetrates to the surface of the substrate of the window material through the coating film. This may accelerate the peeling of the coating film further. At the portion where the coating film is peeled off, the quartz glass is exposed to halogen-containing plasma, so that SiO.sub.2 is generated in a form of fine particles.
In order to prevent a coating film from being cracked due to the thermal stress, a method for forming a metal aluminum coating film and Al.sub.2 O.sub.3 coating film on a quartz glass substrate has been suggested (see Japanese Utility Model Publication No. 61-13555). However, this method causes problem that the semiconductor wafer treated inside the chamber is not sufficiently heated because the metal aluminum coating film is opaque and cuts off the light from the lamp heater.
Methods for coating the substrate of the window material with zirconia, alumina, or mixed oxides thereof have been also suggested (see Japanese Laid-Open Patent Publications Nos. 53-146717 and 54-3118). However, in these inventions, the coating film is crystalline which is hard and brittle. Therefore, when the semiconductor wafer is heated or the temperature increases by plasma, cracks and pin holes are easily produced in the coating film. Through the cracks or pin holes, halogen-containing plasma may penetrate to etch the substrate of the window material.
The present invention has been conducted to solve the above-described problems, and the objective thereof is to provide a coating film which stably exhibits excellent resistance to halogen-containing gas corrosion and halogen-containing plasma corrosion; a laminated structure coated with the same, for example, a window material for a vacuum chamber; and a method for producing the same.
Another objective of the present invention is to provide a laminated structure, for example, a window material for a vacuum apparatus, provided with a coating film which will not become cracked or peeled off due to the thermal influence with high transparency.